Medical imaging apparatus such as digital fluorography systems are well known in the art. For imaging moving objects, such systems operate in a dynamic mode of operation in which a video readout is used. To avoid flicker in the image, the video readout consists of 25-30 frames per second. The X-ray exposure in dynamic imaging systems may be continuous or pulsed, as described in U.S. Pat. No. 4,555,728.
Angiography is a commonly used procedure. Typically, a catheter is guided, through blood vessels, from an insertion site to a target site where a given task is to be carried out. The given task may include, for example, local injection of contrast-enhancing material or expansion of a particular artery using a "balloon" catheter. The position of the surgical instrument during surgery is typically monitored based on dynamic, real time, images of relevant body portions, i.e. body portion en route between the insertion site and the target site. To ensure quick, efficient and accurate guidance of the catheter to the target site, each dynamic image is preferably shown superimposed with a "road-map" image of the corresponding relevant body portion. The road map image outlines the structure, e.g. the blood vessel configuration, of the relevant body portion.
In a prior art system, the road map is constructed based on a static x-ray transmission image of the relevant body portion, produced after injecting contrast-enhancing material into blood vessels of the relevant body portion. The contrast-enhanced blood vessels typically appear as generally darker regions of the static base image. A dynamic, road mapped, image is then formed by subtraction of the blood-vessel-emphasized image from the dynamic image of the relevant body portion. Since the two images differ substantially only in the blood-vessel-enhanced regions, the subtracted image includes essentially only an outline of the blood-vessel configuration in the relevant body portion superimposed with a dynamic image of the surgical instrument. It is appreciated that the quality of the road mapped image is highly dependent on the quality of the static blood-vessel image from which it is formed and on the correlation between the blood-vessel image and the dynamic image of the treated body portion.
According to one existing method, the blood-vessel-emphasized image is constructed from a series of frames, e.g. a series of frames of a dynamic image, which are produced following an injection of contrast-enhancing material. Since the injected contrast-enhancing material travels with the bloodstream, different frames generally emphasize different blood vessel regions. The different frames may be unfiltered frames or frames which are filtered, for example, using recursive filtering, whereby each frame depends to a predetermined extent on the content of preceding frames.
To obtain a complete image of all the emphasized blood-vessels the series of frames may be combined in accordance with a maximum opacity criterion, whereby each pixel in the combined image is taken from the blood-vessel-emphasized-frame having the highest opacity at that pixel. It has been found that although the maximum opacity criterion enables substantially full coverage of any body region, this technique results in loss of detail, particularly in finer blood-vessels regions. This may be due to slight shifts in the pixel-position of the vessels, between frames, and/or due to inability of discriminating, electronically, between the finer blood vessels and arbitrary noise in the image.